Meanwhile, Russian Foreign Minister Sergei Lavrov called for "urgent measures" to de-escalate the Ukraine crisis, in a telephone conversation with US Secretary of State John Kerry.

The country's foreign ministry said Mr Lavrov "stressed the need for urgent measures to de-escalate the situation in Ukraine, above all to stop military operations against protesters and to put an end to the aggressive activities of the ultra-nationalist Pravy Sektor."

There was no sign on Saturday that the separatists were adhering to an agreement struck in Geneva last week by the US, EU, Russia and Ukraine.

Under it, all the separatists were required to disarm and withdraw from their occupations of public buildings in eastern Ukraine.

Instead, the head of the self-proclaimed Donetsk Republic vowed to go ahead with a referendum on independence for the separatist region in eastern Ukraine next month.

The United States and the other leaders of the Group of Seven (G7) major economies agreed on Friday night to impose more sanctions on Russia over the crisis in Ukraine.

According to Reuters, the US is expected to impose sanctions on 15 Russian individuals and other "entities" on Monday.

Reuters sources said US sanctions would be focused on so-called 'cronies' of Russian President Vladimir Putin while EU sanctions would mostly target those responsible for unrest in eastern Ukraine.

A statement from the G7 said the group was prepared to move to broader sanctions on Russia, including 'sectoral measures', should circumstances warrant.

It said Russia had taken no concrete actions to comply with the Geneva accord and had continued to escalate the crisis, but the door remained open to negotiation.

A European Union official said that representatives of EU governments would meet on Monday to discuss further sanctions.

Meanwhile, a group of international observers continued to be held hostage in eastern Ukraine.

Russia said on Saturday it would do everything in its power to get them released.

An OSCE team was on its way to the area to negotiate, but the separatists who are holding them indicated they would only do so in exchange for their comrades who are being held by Ukraine's security forces.

In Lithuania, around 150 troops from the 173rd Airborne Brigade landed at the Siauliai air base.

The American troops will participate in a series of exercises throughout the year, according to the Lithuanian defence ministry.

Another company of soldiers arrived in Poland on Wednesday and in Latvia on Friday and a further 150 are due in Estonia on Monday.